Please Pass the Salt: Using Oil Fields for the
Disposal of Concentrate from Desalination Plants

Jean-Philippe Nicot, Ali H. Chowdhury, and Robert
E. Mace

Abstract:

The demand for fresh water in Texas will increase
in the future owing to population growth and because conventional sources
such as surface water and groundwater will not be sufficient to cover
needs. Desalination of brackish water is a viable alternative being actively
pursued by the State, whose technology of choice includes reverse osmosis
and electrodialysis. Feed water is processed to produce a permeate that
typically augments other water sources. In the process, a concentrate
is created that collects all salts rejected from the permeate. The concentrate’s
salinity ranges from 2 to 5 times that of the feed water. Several options
are available for disposing of desalination concentrates, such as discharge
to the ocean in coastal communities or discharge to a sewer system. Another
promising possibility is deep-well injection. In Texas, the oil and gas
industry has been reinjecting saline produced waters into the subsurface
for years. A combination of deep-well injection and reinjection is attractive
for Texas, where oil and gas fields are plentiful. Formation pressures
in oil and gas reservoirs have been greatly lowered because of past production,
creating an opportunity for injecting foreign fluids at a lower cost.
However, legal and technical issues can arise.

A high level study was undertaken to understand challenges and opportunities
offered by well disposal. Potential injection of concentrate in depleted
fields raises the following questions: (1) Is the maximum authorized injection
pressure high enough to overcome low permeability in some reservoirs and
adequate to take up the entire concentrate stream? (2) Are environmental
downhole conditions, especially pressure and temperature, conducive to
mineral precipitation within the concentrate stream? (3) Is the formation
water compatible with the concentrate stream? (4) Is the salinity shock
prone to mobilizing formation fines and clays as they contact water of
smaller ionic strength and/or different ionic makeup? (5) Is the current
legal environment adequate? For a more realistic analysis, we selected
areas from six sedimentary basins across Texas: the Anadarko Basin in
the Texas Panhandle, the Permian Basin, the East Texas Basin, the Fort
Worth Basin, the Maverick Basin at the Mexico-US border, and the south
part of the Gulf Coast Basin.

Despite some differences, the six analysis areas show a consistent picture.
They all have a history of fresh-waterflooding, especially in their early
production periods. We also analyzed the distribution of injection flow
rates. Achievable injection rates are not on average historically high,
which is confirmed by the low-permeability values of the Paleozoic formations.
The East Texas and Gulf Coast reservoirs have higher permeability and
subsequent maximum potential injection rates. Multiple wells will be needed
to accommodate the desalination concentrate stream of a typical plant.

To analyze the impact of changes in environmental conditions, we used
a batch geochemical code in combination with a Monte Carlo approach. We
sampled the formation-water and brackish-water data sets multiple times
and mixed the selected samples in different proportions. Lack of detailed
chemical analyses did not allow for a thorough study of all scale-forming
minerals, barite in particular. However, the scaling tendency by calcite
and gypsum is not outside of that typically encountered and dealt with
by the oil and gas industry. We also analyzed the impact on clay of fresher-water
injection. Water sensitivity of the clayey material can be accommodated
using operational solutions such as pretreatments with appropriate chemicals
or buffer solutions.

Injection of desalination concentrates and produced waters from oil and
gas fields naturally falls under the jurisdiction of Underground Injection
Control regulations (UIC) under Class I (injection of hazardous and nonhazardous
wastes beneath the base of usable-quality water), Class II (disposal of
saltwater and other fluids co-produced with oil and gas), and Class V.
Permitting the injection of concentrate could be made easier through general
permitting of a special non-hazardous Class I injection well. Class II
wells can be used only if the concentrate aids in hydrocarbon production.
It may also be possible to create a special category of Class I or Class
V injection well. There are also potential federal solutions, but changing
rules at the federal level has been an onerous and thus far unsuccessful
endeavor. In short, it is technically feasible to inject concentrate into
oil and gas fields and there are several options for making the permitting
of concentrate disposal wells easier and more affordable.